A laser printer has a removable toner cartridge disposed therein for providing a supply of toner. The removable toner cartridge includes a photoconductive drum rotatably supported in a cleaner housing and a developer roll rotatably supported in a developer housing, which is slidably received in the cleaner housing. The developer roll transfers toner from the developer housing to the photoconductive drum when both are rotating to print a latent image on a recording medium.
Users of laser printers desire to know when the level of the usable toner in the removable toner cartridge is reaching the usable level at which quality printing will not occur. Various systems for sensing the amount of toner remaining in the removable toner cartridge have previously been suggested.
One sensing system has employed an optical sensor peering through a window defining a portion of a toner sump wall in the removable toner cartridge in which the toner is contained. When the optical sensor sees light through the sump window, a signal is produced to indicate to the user that the amount of remaining toner is low. However, this signal does not provide any indication of the level of the toner remaining in the removable toner cartridge but only that it is lower than a predetermined value. Because users print different quantities of sheets of a recording medium during a fixed period of time such as a day, for example, the indication of a low toner level might be satisfactory for a user printing a relatively large number of sheets of a recording medium but not satisfactory for a user printing a relatively small number of sheets of a recording medium during the same time period or vice versa.
Another sensing system has employed a torque sensing mechanism for sensing the torque driving a stirring paddle for the toner. When the torque required to stir the toner decreases below a minimum value, the torque sensing mechanism signals to the user of the laser printer that the level of the toner is low. The torque at any time during the cartridge life is related to the level of the remaining toner. Thus, this system is used to estimate the level of the remaining toner in the cartridge. However, this system is complex, relatively expensive, and can fail from a number of different causes.
Furthermore, in this torque sensing mechanism, the range of measurement is a relatively small percentage of the toner total load. Therefore, only about 30% of a full toner load is within the toner level measuring range of the torque sensing mechanism. Thus, the torque sensing mechanism does not enable measurement of substantially the entire range of the toner load.
U. S. Pat. No. 5,802,419 to Sakurai et al mentions various types of detecting mechanisms capable of detecting whether the amount of toner remaining in the removable toner cartridge is no more than a predetermined value. These include a mechanism employing a capacity detecting system, a mechanism employing a magnetic sensor, a light transmission type mechanism, and a mechanism that detects the weight of the toner. However, there is no discussion of how the weight of the remaining toner would be ascertained, and there is no suggestion that such mechanism is capable of determining the level of toner from detecting the weight of the toner. There also is no suggestion of using the detected weight of the toner to determine its level at any time.